When a speech/audio signal is transmitted in a mobile communication system or a packet communication system typified by Internet communication, compression/encoding technology is often used in order to increase speech/audio signal transmission efficiency. Also, in recent years, a scalable encoding/decoding method has been developed that enables a good-quality decoded signal to be obtained from part of encoded information even if a transmission error occurs during transmission.
One above-described compression/encoding technology is a time-domain predictive encoding technology that increases compress ion efficiency by using the temporal correlation of a speech signal and/or audio signal (hereinafter referred to as “speech/audio signal”). For example, in Patent Document 1, a current-frame signal is predicted from a past-frame signal, and the predictive encoding method is switched according to the prediction error. Also, in Non-patent Document 1, a technology is described whereby a predictive encoding method is switched according to the degree of change in the time domain of a speech parameter such as LSF (Line Spectral Frequency) and the frame error occurrence state.    Patent Document 1: Japanese Patent Application Laid-Open No. HEI 8-211900    Non-patent Document 1: Thomas Eriksson, Jan Linden, and Jan Skoglund, “Exploiting Inter-frame Correlation In Spectral Quantization,” “Acoustics, Speech, and Signal Processing,” 1996. ICASSP-96. Conference Proceedings, 7-10 May 1996 Page(s): 765-768 vol. 2